European Patent Document EP 0956921 discloses a cutting insert of the above-mentioned type for end milling. Advantages of this cutting insert are, among others, that the same is easy-cutting and provides uniform chip formation and good chip release along the entire length of the main edge, at the same time as the main edge has a good strength and a long service life, as a consequence of the clearance angle of the edge being constant along the entire edge length. Another advantage is that not only the functional clearance of the main edge, but also of the wiper edge in relation to the surfaces generated in the workpiece, becomes reliable. As such, early wearing of the clearance surfaces of the cutting edges is avoided. However, the last-mentioned advantage has been gained at the price of the disadvantage that the cutting insert includes only two usable cutting edges.
Within the field of cutting or chip removing machining, there is a constant aim to design the replaceable cutting inserts of the tools to have as many usable cutting edges as possible. In this respect, end-milling inserts are no exception. Hence, several proposals have been presented to form end-milling inserts with as many as four alternately usable cutting edges, by giving the cutting inserts a square or quadratic basic shape. See, for instance, European Patent Document EP 0787051, U.S. Pat. No. 5,597,271, International Patent Publication No. WO 96/35538, and U.S. Pat. No. 5,810,521.
However, the task of designing four-edged end-milling inserts involves a number of awkward weighings and problems because of the complicated space geometry of the milling-cutter tools and cutting inserts, the most difficult one to master of which is the design of the different clearances of the cutting insert. Thus, it is not only required that the pair of forwardly indexed, active cutting edges of the cutting insert, i.e., a main edge and a wiper edge running at an angle to the same, but also the inactive wiper edge in the extension of the active main edge, as well as the inactive main edge in the extension of the active wiper edge, obtain an acceptable clearance from the generated surfaces. More precisely, the active wiper edge should have a good tangential or rotational clearance so far that the clearance surface being rotationally behind the edge does not contact the plane bottom surface being machined by the same, at the same time as the inactive main edge in the extension of the wiper edge has to have a radial clearance in order not to contact the same surface. In an analogous way, the active main edge, which generates a shoulder or shoulder surface at an angle of 90° to the bottom surface, has to have a tangential clearance so far that the clearance surface being rotationally behind must not contact the shoulder surface, at the same time as the inactive wiper edge in the extension of the main edge neither is allowed to contact the shoulder surface.
Elementary, geometrically working solutions to the above-mentioned problems have been presented in European Patent Document EP 0787051 and U.S. Pat. No. 5,597,271. More precisely, the cutting insert designs disclosed in these two documents are based on the idea of countersinking the wiper edge toward the underside of the cutting insert and in relation to a main edge positioned along one and the same clearance side. In such a way, the wiper edge can, on one hand, work with acceptable clearance from the bottom surface in a state in which the cutting insert is tipped-in into a positive axial angle in the appurtenant milling cutter body, and on the other hand—after indexing of the cutting insert—assume an inactive state in which it does not contact the shoulder surface. This effect is attained by the fact that the cutting insert is held tipped-in at a negative radial angle. For the sake of clarity, the axial and radial angles are determined by the solid geometrical location of a support surface formed in the milling cutter body and against which the underside of the cutting insert is pressed.
Notwithstanding the fact that the two mentioned documents provide theoretical provisions about how the basal complex of geometry problems should be solved, the two solutions are insufficient in certain respects. Thus, in the basic designs thereof, the two cutting inserts are concretely described having straight main edges, and in addition to this, it is incidentally indicated the possibility of giving the main edges a generally curved shape. However, in both cases, the connections of the main edges to the wiper edges are of a kind that is not suitable for practical cutting-insert manufacture.
In this connection, cutting inserts of the kind in question are manufactured by compression moulding of a powder mass into a press body, which subsequently is sintered, for instance while forming cemented carbide. The compression moulding takes place in a tool that includes, on one hand, a die, the shape of which determines the shape of the clearance sides of the cutting insert, and on the other hand two stamps, which give the upperside and underside of the cutting insert the desired shape. For allowing a pressing operation, on one hand, to be carried out by accurate precision, and on the other hand to enable release of the press body from the moulding tool, it is required that the different part surfaces, which together define the overall shape of the cutting insert, do not include any sharp or abrupt transitions between one another. Neither must any individual part surface have too narrow a clearance angle, because otherwise the press body may be damaged when it is to be released. For these reasons, it is important that the different part surfaces are smoothly rounded.
The present invention aims at obviating the shortcomings of the end-milling inserts previously disclosed in European Patent Document EP 0787051 and U.S. Pat. No. 5,597,271, and at providing an improved end-milling insert.
An object of the invention to provide a practical, well-functioning, four-edged end-milling insert, which combines the advantages of the easy-cutting, curved main edges with theoretical geometry solutions. In other words, the invention aims at providing an end-milling insert that:
a) has four easy-cutting and strong main edges, which provide good chip formation and flexible chip evacuation, and which become worn evenly and slowly while ensuring a long service life of the cutting insert,
b) has four wiper edges that, on one hand, in the active state thereof, have a good surface-wiping effect on a generated bottom surface, and on the other side, in the inactive state thereof, have a reliable clearance from a shoulder surface generated by an active main edge, and
c) ensures that the inactive main edge, in the extension of an active wiper edge, obtains a good radial clearance from the wiper edge.
Another object of the invention is to provide an end-milling insert, the different part surfaces of which are smoothly rounded to allow a rational and efficient series manufacturing of the cutting insert.
Yet another object of the invention is to provide an end-milling insert that has a proportionally large underside or base surface in order to ensure a stable fixation of the same.